This invention relates to an impressed current cathodic protection system for obtaining anti-electrolysis action at deep well submersible pumping equipment to counter the effects of electrolytic corrosion.
In pumping fluid from a deep well, such as an oil well, a conventional practice is to position a pumping assembly (usually including an electric motor, a pump driven by the motor, and a seal section) in the well near the bottom thereof. Electrical energy is supplied from the surface to the downhole pumping assembly via a power cable and the pumped fluid is conveyed from the well to the surface by means of tubing. Customarily, the motor is positioned below the pump, with the seal section effectively interfacing the two. The entire pumping assembly will therefore normally be immersed in the well fluid. The seal section retains within the motor a fluid which serves both as a lubricant and as a coolant for the moving parts of the motor. The seal section also prevents the well fluid from entering the motor, while at the same time balancing interval submergence pressures. In addition, the seal section carries the thrust of the pump during operation.
Unfortunately, the fluid in a deep well may include components that will corrode the metal surfaces of the pumping assembly that are in contact with the well fluid. Usually, this would include the external surfaces of the metal housings in which the pump, motor and seal sections are encased, plus the internal surfaces of the pump. Examples of corrosive components in the well fluid are brine and disolved carbon dioxide. Such components will attack and corrode any metal surface. The corrosion is caused by electrolytic action, the corrosive solution constituting an electrolyte through which electrons flow toward the metal surfaces.
The present invention overcomes the electrolytic corrosion problem, which plagues downhole pumping equipment, by effectively reversing the electrolysis process so that electrons will flow away from the metal surfaces to be protected. This is achieved by making the metal surfaces cathodic with respect to an anode and impressing current flow from the anode to the cathodic elements. Of course, current flow and electron flow are in opposite directions. Current flows from anode to cathode, while electrons flow from cathode to anode.